What does presence knob actually do?

[JumpMarine]

I can't add to the great expaination that Lew gave, but on my JCM I run my presence around 2 to 4 o'clock other wise the tone can be a bit flabby below that.

[Kent S.]

Originally posted by Lewguitar
Do you want to know what it does or how it does it? Most people would say that it increases the highs but doesn't just boost them: it opens them up and makes them more lively.

The presence control is part of the negative feedback circuit of an amp.

After the guitar has been amplified by the preamp section of your amp (the small 12ax7 style tubes) a small part of that signal is taken, the phase reversed, and then reintroduced into the phase invertor/driver section...that's the Negative feedback circuit: the small out of phase signal.

That signal that is reintroduced is out of phase with the main audio signal and what it does is reduce distortion and tighten up the tone. It sort of "clamps down" on it and keeps the bass from getting flubby and makes the tone a little thinner...just like when you make a mistake instaling a pickup and accidently wire one out of phase so that when when you combine the two pickups the resulting tone is thin.

What the presence control does when you turn it up is send JUST the treble frequencies that are part of the negative feedback circuit to ground. The more you turn the presence control up the more treble from the negative feedback circuit is sent to ground and NOT reintroduced into the main audio signal.

Since those out of phase treble frequencies are now going to ground, they can't clamp down and dampen the primary treble frequencies of the main audio signal.

The result is that the main audio signal gets brighter and livlier.

And thats's how it works.

Lew

Excellent reply Lew, i would like to add one thing however, what Lew has described is the traditional control ... and it is interesting in that it does effect to way the power amp feels and the way it response a bit gain wise (as far as clipping and compression).A very cool personality change for your power amp. However, on some newer amps the presence control is located in the preamp, and is basically and extra treble control voiced higher, sometimes acting as a balance between the highs and lows. So on some amps it's basically preamp contained eq stage ... on some amps it simply adds higher voiced treble frequencies, on others it acts as a *tone control* after your eq, so after your tone is dialed in you can brighten up, or darken up your tone as needed, without altering your basic tone (kinda tuning the highs/lows to the room).
Lew is 100% dead on correct, but there is also a preamp version that is more of an eq control. Just thought i'd toss that distinction out there ...

[juvenal]

What Lew said...

You can play with the cap value on the presence pot to shift the frequency it adjusts so that it almost sounds like you're dialing in gain (hence, the negative feedback connection).

[StefanM]

Lew gave the more detailed answer than I can.

Basically, presence is higher than 'treble' - the high highs, but is done at the power amp section rather than at the preamp. On my amp, at least, higher settings give the impression of more headroom and makes it more open sounding - lower settings make it darker, more compressed. Boogie says 'more vocal'. With higher gain settings you need to watch it to avoid microphonics.



BTW Why does 'quick reply' never work for me??

[Chi3f]

damn i'd like to try out the savage 120

[Young Angus]

Originally posted by Push to Shuv
whats an ENGL?

lol

Check this out and be wowed!

http://www.engl-amps.com/

[Push to Shuv]

whats an ENGL?

You're Welcome! Lew

[Simon_F]

Originally posted by Lewguitar
Do you want to know what it does or how it does it? Most people would say that it increases the highs but doesn't just boost them: it opens them up and makes them more lively.

The presence control is part of the negative feedback circuit of an amp.

After the guitar has been amplified by the preamp section of your amp (the small 12ax7 style tubes) a small part of that signal is taken, the phase reversed, and then reintroduced into the phase invertor/driver section...that's the Negative feedback circuit: the small out of phase signal.

That signal that is reintroduced is out of phase with the main audio signal and what it does is reduce distortion and tighten up the tone. It sort of "clamps down" on it and keeps the bass from getting flubby and makes the tone a little thinner...just like when you make a mistake instaling a pickup and accidently wire one out of phase so that when when you combine the two pickups the resulting tone is thin.

What the presence control does when you turn it up is send JUST the treble frequencies that are part of the negative feedback circuit to ground. The more you turn the presence control up the more treble from the negative feedback circuit is sent to ground and NOT reintroduced into the main audio signal.

Since those out of phase treble frequencies are now going to ground, they can't clamp down and dampen the primary treble frequencies of the main audio signal.

The result is that the main audio signal gets brighter and livlier.

And thats's how it works.

Lew

That's excellent Lew!! Thanks!!

Originally posted by Young Angus
Cheers Lew, i got a bit out of that

If only i understood things a little better...one day *sigh*

Fortunately there's really no need to understand "how" an amp works to dial in a great tone from your amp! Thanks! Lew

[Young Angus]

Originally posted by PUCKBOY99
Is that ENGL still not working out for you? That really is a shame, man.

Im not sure if the bloody ENGL is working for me or not, i havent had a chance to crank it much although i got it up half way the other day and it sounded pretty nice! Only for a second though.

But i still think theres something not right with it, ive gotta get it checked out again soon.

[Young Angus]

Cheers Lew, i got a bit out of that

If only i uderstood things a little better...one day *sigh*

Do you want to know what it does or how it does it? Most people would say that it increases the highs but doesn't just boost them: it opens them up and makes them more lively.

The presence control is part of the negative feedback circuit of an amp.

After the guitar has been amplified by the preamp section of your amp (the small 12ax7 style tubes) a small part of that signal is taken, the phase reversed, and then reintroduced into the phase invertor/driver section...that's the Negative feedback circuit: the small out of phase signal.

That signal that is reintroduced is out of phase with the main audio signal and what it does is reduce distortion and tighten up the tone. It sort of "clamps down" on it and keeps the bass from getting flubby and makes the tone a little thinner...just like when you make a mistake instaling a pickup and accidently wire one out of phase so that when when you combine the two pickups the resulting tone is thin.

What the presence control does when you turn it up is send JUST the treble frequencies that are part of the negative feedback circuit to ground. The more you turn the presence control up the more treble from the negative feedback circuit is sent to ground and NOT reintroduced into the main audio signal.

Since those out of phase treble frequencies are now going to ground, they can't clamp down and dampen the primary treble frequencies of the main audio signal.

The result is that the main audio signal gets brighter and livlier.

And thats's how it works.

Lew

[y2stevo]

i always thought presence adjusted the brightness...

[PUCKBOY99]

NOTHING LIKE A GOOD KNOB !!!

Is that ENGL still not working out for you? That really is a shame, man.

As far as the presence knob: I would imagine it's semi-universal, but on my Mesa 20/20, it basically shaped or blended the mids a little better...kind of like the tone knob on the guitar.